Apparatus for electrostatic printing

ABSTRACT

An electrostatic printer employs a metallic drum coated with a semiconductor film. The resistance of the film is varied from its high resistance or insulating state to its low resistance or conducting state in response to a scanning laser beam. A DC voltage source is connected to the metallic drum. The voltage is conducted through the semiconductor and appears at the surface in those regions where the laser beam impinges on the semiconductor. The semiconductor is then dusted with ink particles which cling to the regions where the voltage appears. Printing is accomplished by transferring the ink particles to a document. Multiple copies may be made by employing a semiconductor material exhibiting a memory characteristic.

United States Patent Klose et a1.

[54] APPARATUS FOR ELECTROSTATIC PRINTING [72] Inventors: Peter H. Klose, Troy; Stanford R. Ovshinsky, Bloomfield Hills. both of Mich.

[731 Assignee: Energy Conversion Devlces, Inc., Troy,

Mich.

[22] Filed: Jan. 7, 1970 [21] Appl. No.: 1,265

[52] U.S. Cl ..355/3, 96/ 1, 346/74, 340/173 [51] Int. Cl. ..G03g 15/00 [58] Field of Search ..96/1, 1.4; 355/3, 17; 346/74 P, 74 ES; 340/173 LM [56] References Clted UNITED STATES PATENTS 2,764,693 9/1956 Jacobs et a1. ....250/214 3,355,289 11/1967 Hall et a1. ..96/1.4 3,396,235 8/1968 Button et al. ..96/1 X [4 1 May 2, 1972 Kazan et al.

Herrick, Jr. et a] ..96/1.5 X

Primary Examiner-Samuel S. Matthews Assistant Examiner-Robert P. Greiner Attorney-Edward G. Fiorito ABSTRACT An electrostatic printer employs a metallic drum coated with a semiconductor film. The resistance of the film is varied from its high resistance or insulating state to its low resistance or conducting state in response to a scanning laser beam. A DC voltage source is connected to the metallic drum. The voltage is conducted through the semiconductor and appears at the surface in those regions where the laser beam impinges on the semiconductor. The semiconductor is then dusted with ink particles which cling to the regions where the voltage appears. Printing is accomplished by transferring the ink particles to a document. Multiple copies may be made by employing a semiconductor material exhibiting a memory characteristic.v

17 Claims, 6 Drawing Figures DA 74 PIOCESSl/VG 5 VS 75M APPARATUS FOR ELECTROSTATIC PRINTING This invention relates to printers of the type generally known as electrostatic printers. An electrostatic image is normally formed on the surface of a printing plate. The plate is dusted with particles of ink which cling to the charged areas. The ink particles are subsequently transferred to a document and the printing plate is cleaned in preparation for the next printing cycle.

One form of printing plate employs a photoconductive surface applied to a supporting structure. The surface of the photoconductor is charged to a high potential, typically with a corotron, such as that described in the text Xerography and Related Processes" edited by J. Dessauer and H. Clark, published by Focal Press Limited, I965. The photoconductor material once charged must normally be kept in the dark until a light image can be applied. Those areas of the photoconductor which are illuminated conduct the charge down to the supporting structure. The unexposed area of the photoconductor sustains the original charge for normally only a short period of time during which it must be developed by dusting with ink particles. The original charge also tends to migrate toward the outer boundary of the unexposed areas of the photoconductor thereby distorting the printed image.

The principle object of the present invention is to provide new and improved methods and apparatus for producing an electrostatic image which can be used to print single or multiple copies from a single image. A printing plate is constructed with a conductive base having a coating composed of semiconductor material which exhibits variable resistance in response to energy applied thereto. Such materials may be, for example, the materials disclosed in U.S. Pat. No. 3,271,591, granted on Sept. 6, 1966 to Stanford R. Ovshinsky, and generally known as amorphous semiconductors. Other suitable semiconductor materials may be used in cooperation with an appropriate source of energy such as a laser, electron beam, heat, light, or voltage source. The source of energy is directed onto the printing plate to change the semiconductor material at selected regions from a high resistance condition to a low resistance condition.

In accordance with the present invention a source of potential is applied to the conductive base. When the semiconductor coating is in its high resistance condition the potential applied to the conductive base is efiectively insulated from the outer surface of the printing plate. In order to insure that the outer surface of the printing plate is initially at a substantially zero potential a grounding brush, for example, may be used to sweep the surface at the beginning of the printing cycle. 5

The semiconductor material employed in the present invention may exhibit a memory characteristic, switching between two stable resistance states, or a non-memory characteristic, shifting temporarily from a high resistance stable state to a low resistance condition, and then returning automatically to the high resistance state. The semiconductor material exhibiting the memory characteristic retains its low resistance state until it is switched back by the reapplication of energy. While the semiconductor is in its low resistance state the potential applied to the conductive base is transmitted through the semiconductor to the outer surface of the printing plate. Accordingly selected regions of the printing plate exhibit a positive or negative potential depending upon the polarity of the potential applied to the conductive base. Other regions of the printing plate, where the semiconductor material is in its high resistance state, are effectively insulated from the potential applied to the conductive base and therefore reside at approximately ground potential. In this manner an electrostatic image is formed corresponding to the pattern of energy applied to the semiconductor material. The electrostatic image may be developed by dusting with ink particles which cling to the regions of the, semiconductor material residing in the low resistance state. The ink particles are then transferred to a document where they may be fixed to the document by the application of heat, for example. When a semiconductor material exhibiting a memory characteristic is employed in the present invention, it may be developed once again by dusting with ink particles to produce another copy. Any charge that may be carried off during development, or during transfer of the ink to the document, is replenished by transmission of the potential from the conductive base through the low resistance regions of the semiconductor materials. Therefore, many copies can be produced during successive print cycles.

In order to prepare the memory semiconductor material for printing a new image, it is preferable to reset the material into the high resistance state. This may be accomplished by exposing the material to an energy source such as a laser, electron beam, R.F. heating source, quartz heat lamp or voltage source. Following reset, the semiconductor material may be swept with a grounding brush to eliminate any residual charge prior to applying a pattern of energy during the next printing cycle.

If a non-memory semlconductive material is employed in this invention, the operation is similar to that described above for the memory material. At the beginning of the printing cycle the semiconductor material is in its high resistance or insulating statefl'he material is shifted temporarily to its low resistance condition by the application of a pattern of energy, which may be, for example, in the form of light passing through a transparency, or in the form of a scanning laser beam pulsed on and off in response to signals from a data processing system such as that disclosed in US. application Ser. No. 828,859 filed May 29, 1969 entitled HIGH SPEED PRINTER OF MULTIPLE COPIES FOR COMPUTER OUT- PUT INFORMATION by Stanford R. Ovshinsky, Ronald G. Neale and Edgar J. Evans. The material shifts to its low resistance condition for a short interval of time since this condition is an unstable condition normally requiring a holding current to sustain it. During this interval the potential applied to the conductive base is transmitted through the semiconductor to the surface thereof where it is trapped upon the return of the semiconductor material to its high resistance state. The trapped potential forms a layer of charge on the upper surface of the semiconductor material thereby contributing to the formation of an electrostatic image corresponding to the scanning pattern of the pulsed laser beam. The electrostatic charge pattern is developed by dusting with ink particles and the particles are then transferred to a document and fixed thereon. In order to prepare this semiconductor material for the next printing cycle a grounding brush may be employed to sweep the trapped charge from the surface of the semiconductor.

From the above description it can be seen that the present invention does not require the use of a high voltage source such as a corotron to deposit a charge on the surface of the printing plate, either prior to the electrostatic image fonning process, or during development thereof. Also, the semiconductor material of the present invention need not be maintained in a dark environment but may be exposed to ambient or spurious light introduced from the surrounding environment.

Another feature of the present invention is its ability to maintain the electrostatic charge pattern for a relatively long period of time, without, migration of the charge from the regions of the semiconductor material switched to the low resistance state. This latter feature of the present invention permits the development of an electrostatic charge pattern having a grey scale. The amount of electrostatic charge produced at any point on the printing plate can be varied by the level of energy applied thereto. The charge raised to the surface of the semiconductor material has very little mobility, and accordingly remains in the same spot on the surface of the printing plate. Upon dusting with ink particles varying amounts of particles cling to surface dependent upon the charge at each point. When the ink particles are transferred to a document, varying shades of grey can be observed as a function of the level of energy originally applied to the semiconductor materi- Still another advantage of the present invention is the low level of energyrequired to switch the semiconductor material,

particularly the non-memory material which need only become conductive for a short interval of time in order to trap charge at its surface.

Other objects, advantages and features of this invention will become apparent to those skilled in the art upon reference to the accompanying specification, claims and drawings in which:

FIG. 1 is a diagrammatic illustration of one embodiment of an electrostatic printer according to the present invention;

FIG. 2 is an enlarged partial view of the printing plate in FIG. 1, employing semiconductor material exhibiting a memory characteristic;

FIG. 3 is an enlarged partial view of the printing plate in FIG. 1, employing semiconductor material exhibiting a temporary or nonmemory characteristic;

FIG. 4 is a partial view of another embodiment of the present invention employing slide wires as a source of energy for varying the resistance of the semiconductor material;

FIG. 5 is a partial diagram of another embodiment of the present invention employing a cathode ray tube having slide wires mounted in the face thereof, for conducting energy from the electron beam to the semiconductor material coating on the printing plate; and

FIG. 6 is a partial diagram illustrating another embodiment of the present invention employing a cathode ray tube located adjacent to the surface of a printing plate so that the electron beam provides energy sufficient to vary the resistance of the semiconductor material on the surface of the printing plate.

In FIG. 1 the printing plate designated generally as 10 is shaped in the form of a drum having a conductive base 12 and a semiconductor coating 14. The conductive base 12 may be composed of any metallic substance that exhibits sufficient rigidity to provide a structural support for the printing plate 10, and also exhibits the electrical characteristics of a conductor. Suitable materials for the conductive base 12 are aluminum, stainless steel, and tantalum.

The semiconductor material 14 exhibits a variable resistance in response to energy applied thereto. As will be described in more detail below this change in resistance may be a relatively stable change, or a transient unstable change.

The printing plate 10 is mounted for rotational movement, and is driven by a motor 16. Located about the periphery of the plate 10 are a number of stations for performing operations on the semiconductor coating 14. At a write station 18 a laser beam 20 scans across the printing plate 10 perpendicular to the direction of rotation of the plate 10. A developing station 22 dusts the printing plate 10 with ink particles (not shown in FIG. 1). The ink particles are transferred to a document 26 at a printing station 28. After printing, the ink particles are removed at a wiper station 32. If required, the semiconductor 14 is switched to its original resistance condition at a reset station 34. A grounding station 36 removes any residual charge on the surface of semiconductor 14.

A DC potential source 38 is connected to the conductive base 12. In the embodiment illustrated in FIG. 1 source 38 supplies a positive potential of 400 volts to conductive base 12. However, a range of potentials between 300 and 800 volts can be employed. The potential of source 38 is efiectively insulated from the outer surface of printing plate 10 by the semiconductor material 14 when in its high resistance state.

The same numerals are used to designate the same elements in FIGS. 1 through 6. FIG. 2 illustrates an enlarged section of printing plate 10. Regions 14a illustrate points in the semiconductor material 14 which have been switched to the low resistance state in response to laser beams 20. The positive potential of source 38 travels through base 12, and up through semiconductor 14 at points 14a to the surface of printing plate 10. When the printing plate 10 is passed through the developing station 22 shown in FIG. 1, ink particles designated 40 in FIG. 2 cling to the regions of low resistance 14a. These regions remain in the low resistance state when a memory type semiconductor 14 is employed. Suitable materials exhibiting this characteristic are generally known as amorphous semiconductors and may be found in U. S. Pat. No. 3,271,591 They may comprise tellurium and germanium at about percent tellurium 50 percent germanium in atomic percent with inclusions of some oxygen and/or sulphur. Other compositions may comprise Se Te, ,Ga Se Te, ,M or Se Te, ,Te The thickness of the semiconductor coating 14 may be in the range of 40 to 60 microns. The resistivity of semiconductor 14 when in the high resistance condition may be approximately 10 ohm-cm and when in the low resistance condition the semiconductor 14 may have a resistivity of IO ohm-cm or less.

FIG. 3 illustrates the resultant charge pattern where a nonmemory semiconductor material 14 is employed. A group of charge areas 14b are formed on the surface by varying the resistance of the semiconductor material 14 temporarily to its low resistance condition in response to pulses of energy from laser beam 20. Upon the application of a pulse from laser beam 20 the semiconductor material 14 forms a temporary path of low resistance from the surface of printing plate 10 to the conductive base 12, similar to that shown in FIG. 2. When the semiconductor material 14 returns to its high resistance state the charge 14b is trapped at the surface. Ink particles 40 cling to the trapped charge 14b when the printing plate 10 is passed through the developing station 22 in FIG. 1'. Examples of amorphous non-memory semiconductor material 14 suitable for use in the present invention are se Te S Se S As and S As Se Other compositions are described in.U.S. Pat. No. 3,271,591 (such materials being referred to therein as Mechanism devices).

The compositions referred to above in connection with the memory semiconductor material may also be used as nonmemory semiconductor material by decreasing the intensity of the laser beam 20, or decreasing the pulse width, thereby lowering the Joule heating of the semiconductor material 14 so that it does not completely switch into its low resistance state.

The operation of the electrostatic printer illustrated in FIG. 1 is controlled by a Data Processing System 42 which coordinates the operation of stations 18 and 34, and motor 16 through a group of lines 44-48. A source 54 generates laser beam 20 in response to signals on control line 44. The beam 20 is modulated into a series of pulses by optical shutter 56 controlled by line 45. The combination of source 54 and shutter 56 may be employed to produce a train the laser pulses of varying amplitude, duration and separation. A rotating mirror 58 causes the laser beam 20 to scan across the semiconductor 14 in a direction perpendicular to the rotation of printing plate 10. The speed of the rotating mirror 58 is controlled by a motor 60 in response to signals on line 46. Laser beam 20 may have a wavelength of 1.06 microns, and source 54 may produce 0.1 micro joule/image element 14a or 14b.

By coordinating the rotation of printing plate 10 and mirror 58 with the shutter 56 a pattern of laser energy can be applied to semiconductor 14. Selected regions are switched into the low resistance state thereby establishing an electrostatic charge pattern on the surface of printing plate 10. This pattern is rotated through developing station 22 where it is dusted with ink particles which cling to the regions of positive potential. The developed electrostatic image continues to rotate into the printing station 28 where a continuous form document 26 is fed from a roll 62 around a pair of rollers 64 and 65 which keep the document 26 in contact with the printing plate 10. A corotron 66 attracts the ink from the surface of plate 10 onto the document 26. The inked document 26 passes through a heater 70 which fixes the ink to the paper.

Prior to forming a new electrostatic image the surface of plate 10 is cleaned at wiper station 32, removing any remaining ink particles. The wiper station 30 includes a rotating brush 72, and a source of negative potential 74 in the range between -300 to -500 volts. The source 74 is connected to the brush 72 enabling it to lift the ink particles from the plate 10 more readily. If a memory type semiconductor 14 is employed, and additional copies are desired, the semipermanent electrostatic image generated by the laser during the first cycle is passed through developer station 22 where is it dusted again and another copy is printed at station 28. After the desired number of copies are printed, the memory type semiconductor material 14 is reset at station 34 in response to a signal on line 48. The reset at station 34 may include a source of energy such as an RF heating source, a quartz heat lamp, a voltage source, a laser or an electron beam.

Any residual charge is removed from the surface of plate by a brush 78 connected to ground. Where a non-memory semiconductor material such as that illustrated in FIG. 3 is employed in the electrostatic printer of FIG. 1, the operation is similar to rat described above when using a memory type semiconductor material 14, except that reset station 34 is not required, since the semiconductor material 14 returns to its original resistance state without any external assistance.

In the event gray scale printing is desired, the energy applied to semiconductor 14 is varied at the write station 18. This may be accomplished by varying the intensity level of source 54 or the pulse width produced by shutter 56. Still another way may be by varying the speed of motors 16 or 60 so that the dwell time of the laser beam 20 on any point on the surface of plate 10 can be varied. The semiconductor material 14 changes its resistance as a function of the laser energy applied to plate 10. In a corresponding manner the amount of charge conducted from base 12 to the surface of plate 10 is controlled. The electrostatic charge pattern thus produced is passed through the developing station 22 where ink particles cling to each pointon the surface of plate 10 as a function of the amount of charge residing at such point. Accordingly, after the ink particles are transferred at print station 28, varying shades of gray are observed on the document 26. In the event additional copies are desired, the print cycle is repeated in the manner described above.

Other sources of energy may be employed in addition to the laser beam 20 shown in FIG. 1. A writing station designated 18a in FIG. 4 performs the equivalent function of writing station 18 in FIG. 1. Station 18a includes a'bloclr 80 having a row of slide wires 82 extending therefrom and engaging semiconductor material 14. The slide wires are connected to a plurality of gates 84 through a cable 86. The gates 84 each connect a negative potential source 88 to a different one of the slide wires 82. The gates 84 may be turned on and ofi' by any suitable source of signals on a set of control lines 84a which may be provided by a Data Processing System such as that shown in FIG. 1. The slide wires 82 conduct negative pulses to the surface of semiconductor material 14 thereby establishing a potential gradient across the material 14. The potential gradient may be sufiicient to switch the material 14 permanently to its low resistance stable state. Alternatively, the resistance of semiconductor material 14 may be varied temporarily. When the negative pulses on slide wires 82 terminate, or when the slide wires move to a new location, the positive potential on base 12 extends up through semiconductor material 14 to form the positive charge illustrated at 14b in FIG. 4. By selectively operating gates 84 a train of negative pulses can be generated on slide wires 82 which in turn produce an electrostatic charge pattern on semiconductor material 14. In a manner described with regardto FIG. 1 the printing plate 10 of FIG. 4 can be developed and documents printed therefrom.

FIG. 5 illustrates still another writing station designated 18b which may be employed in the present invention. A pin tube 90 is located adjacent to printing plate 10 so that a row of slide wires 92 are in contact with the plate 10. An electron beam 94 selectively applies charge to wires 92 which in turn vary the resistance of the semiconductor material of plate 10 in a manner similar to that described in FIG. 4. The electron beam 94 may be controlled in any suitable manner in cooperation with the movement of plate 10 so that an electrostatic image is formed on the surface of plate 10. The electrostatic image is developed and copies are printed in the same manner as that described with reference to FIG. 1.

FIG. 6 illustrates another source of energy designated 18c which includes a cathode ray tube 96, having a face 97 located sufficiently close to printing plate 10 so that an electron beam 98 supplies energy to semiconductor 14 changing its resistance to the low resistance condition. The thickness of the semiconductor material 14 can be varied so that it will change its resistance in response to the low energy supplied by beam 98. Additionally, the face 97 may be coated with a phosphor which illuminates in response to the beam 98. The semiconductor material 14 may be selected to exhibit a change in resistance in response to the illumination of the phosphor on face 97. An example of such material is Se with 5 percent arsenrc.

Another modification may be made to the present invention by reversing the polarity of DC sources 38 and 88 in FIG. 4. Also, reset of the memory type semiconductor material 14 can be accomplished by defocusing the laser beam 20 instead of employing a separate reset station 34.

While a single laser beam 20 is illustrated in the system of FIG. 1, multiple laser beams may be used in parallel to simultaneously develop an image on the surface of printing plate 10. Additionally, an entire image may be applied to plate 10 at one time by illuminating the surface thereof through a transparency, or by reflection from a surface containing the image to be copied. Suitable adjustments in the thickness and sensitivity of semiconductor material 14 may be made depending upon the type of electromagnetic radiation source employed in this embodiment of the present invention.

Numerous. other modifications may be made to the various fonns of the invention described above without departing from the spirit and scope of the present invention.

What is claimed is:

1. Apparatus for electrostatic printing comprising:

a printing plate including a conductive base having a coating of a semiconductor material thereon, said semiconductor material being capable of varying its resistance from a high resistance condition to a low resistance condition in response to energy applied thereto:

a substantially constant DC voltage source one pole of which is conductively connected to said base for providing a charge which is conducted through said semiconductor material in those areas where said semiconductor is in said low resistance condition;

writing means for applying energy to selected regions of said coating, in an amount sufficient to vary the resistance of said semiconductor material from said high condition to said low condition to pennit passage of said voltage to the surface of said semiconductor material in said selected regrons;

modulating means for varying the amount of energy applied to other regions of said coating to a level which is insufficient for varying the resistance of said semiconductor material in said other regions to said low condition,

developing means for applying particles to said printing plate which cling to said coating at said selected regions; and 3 transfer means for engaging said printing plate with a record medium to transfer at least a portion of said clinging particles to said record whereby an image of said selected regions is printed on said record.

2. Apparatus as defined in claim 1 wherein said coating has a single stable state in said high resistance conditionand shifts temporarily from said high resistance condition to said low resistance condition in response to said energy applied thereto to permit voltage from said source connected to said base to travel through said coating during said temporary shift thereby trapping said charge on said coating when said coating returns to said high resistance condition.

3. Apparatus as defined in claim 2 further characterized by the addition of a grounding means for removing said charge trapped on said coating after said image is printed on said record.

4. Apparatus as defined in claim 3 further characterized by the addition of cleaning means for wiping said clinging particles from said coating after said image is printed on said record to prepare said material for printing another image.

5. Apparatus as defined in claim 1 wherein said coating comprises a semiconductor material which has two stable states, a high resistance state and a low resistance state, and which is capable of being switched between said states in response to said energy applied thereto.

6. Apparatus as defined in claim 5 further characterized by the addition of means for applying energy to said coating to switch said coating from said low resistance state to said high resistance state.

7. Apparatus as defined in claim 1 wherein said writing means includes a modulated laser beam.

8. Apparatus as defined in claim 1 wherein said writing means includes a modulated electron beam.

9. Apparatus as defined in claim 1 wherein said writing means includes a plurality of slide wires in contact with said coating.

10. Apparatus as defined in claim 9 wherein an electron beam selectively applies a charge to said slide wires.

11. Apparatus as defined in claim 1 wherein said coating is an amorphous semiconductor.

12. Apparatus for electrostatic printing comprising:

a rotatable printing drum including a metal cylinder having an amorphous semiconductor coating applied to the outer surface thereof, said amorphous semiconductor having a bistable characteristic including a high resistance state and a low resistance state;

drive means for rotating said drum;

a constant DC voltage source one pole of which is conductively connected to said base;

writing means for applying a pattern of energy to said amorphous semiconductor at selected regions, said pattern having an energy content sufiicient to switch said coating from said high resistance state to said low resistance state to permit passage of said voltage to the surface of said amorphous semiconductor material in said selected regions;

developing means for dusting said amorphous semiconductor coating with particles of ink as said drum rotates, said ink clinging to said regions of said amorphous semiconductor residing in said low resistance state;

printing means for placing a document in contact with said amorphous semiconductor and transferring at least a portion of said ink particles to said document;

reset means for applying a uniform field of energy across said amorphous semiconductor, said field having an energy content sufficient to switch said amorphous semiconductor from said low resistance state to said high resistance state;

grounding means for removing charge from said amorphous semiconductor coating; and

control means connected to said reset means for selectively rendering said reset means inoperative during certain revolutions of said drum, whereby said developing means applies ink to said coating during said certain revolutions, and said printing means prints a plurality of copies of said image.

13. Apparatus as defined in claim 12 further characterized by the addition of cleaning means for wiping said clinging particles from said coating of said drum.

14. Apparatus as defined in claim 12 wherein said writing means includes a modulated laser beam.

15. Apparatus as defined in claim 12 wherein said writing means includes a modulated electron beam.

16. Apparatus as defined in claim 12 wherein said writing means includes a plurality of slide wires in contact with said amorphous semiconductor coating.

17. Apparatus as defined in claim 16 wherein an electron beam selectively applies a charge to said slide wires. 

1. Apparatus for electrostatic printing comprising: a printing plate including a conductive base having a coating of a semiconductor material thereon, said semiconductor material being capable of varying its resistance from a high resistance condition to a low resistance condition in response to energy applied thereto: a substantially constant DC voltage source one pole of which is conductively connected to said base for providing a charge which is conducted through said semiconductor material in those areas where said semiconductor is in said low resistance condition; writing means for applying energy to selected regions of said coating, in an amount sufficient to vary the resistance of said semiconductor material from said high condition to said low condition to permit passage of said voltage to the surface of said semiconductor material in said selected regions; modulating means for varying the amount of energy applied to other regions of said coating to a level which is insufficient for varying the resistance of said semiconductor material in said other regions to said low condition, developing means for applying particles to said printing plate which cling to said coating at said selected regions; and transfer means for engaging said printing plate with a record medium to transfer at least a portion of said clinging particles to said record whereby an image of said selected regions is printed on said record.
 2. Apparatus as defined in claim 1 wherein said coating has a single stable state in said high resistance condition and shifts temporarily from said high resistance condition to said low resistance condition in response to said energy applied thereto to permit voltage from said source connected to said base to travel through said coating during said temporary shift thereby trapping said charge on said coating when said coating returns to said high resistance condition.
 3. Apparatus as defined in claim 2 further characterized by the addition of a grounding means for removing said charge trapped on said coating after said image is printed on said record.
 4. Apparatus as defined in claim 3 further characterized by the addition of cleaning means for wiping said clinging particles from said coating after said image is printed on said record to prepare said material for printing another image.
 5. Apparatus as defined in claim 1 wherein said coating comprises a semiconductor material which has two stable states, a high resistance state and a low resistance state, and which is capable of being switched between said states in response to said energy applied thereto.
 6. Apparatus as defined in claim 5 further characterized by the addition of means for applying energy to said coating to switch said coating from said low resistance state to said high resistance state.
 7. Apparatus as defined in claim 1 wherein said writing means includes a modulated laser beam.
 8. Apparatus as defined in claim 1 wherein said writing means includes a modulated electron beam.
 9. Apparatus as defined in claim 1 wherein said writing means includes a plurality of slide wires in contact with said coating.
 10. Apparatus as defined in claim 9 wherein an electron beam selectively applies a charge to said slide wires.
 11. Apparatus as defined in claim 1 wherein said coating is an amorphous semiconductor.
 12. Apparatus for electrostatic printing comprising: a rotatable printing drum including a metal cylinder having an amorphous semiconductor coating applied to the outer surface thereof, said amorphous semiconductor having a bistable characteristic including a high resistance state and a low resistance state; drive means for rotating said drum; a constant DC voltage source one pole of which is conductively connected to said base; writing means for applying a pattern of energy to said amorphous semiconductor at selected regions, said pattern having an energy content sufficient to switch said coating from said high resistance state to said low resistance state to permit passage of said voltage to the surface of said amorphous semiconductor material in said selected regions; developing means for dusting said amorphous semiconductor coating with particles of ink as said drum rotates, said ink clinging to said regions of said amorphous semiconductor residing in said low resistance state; printing means for placing a document in contact with said amorphous semiconductor and transferring at least a portion of said ink particles to said document; reset means for applying a uniform field of energy across said amorphous semiconductor, said field having an energy content sufficient to switch said amorphous semiconductor from said low resistance state to said high resistance state; grounding means for removing charge from said amorphous semiconductor coating; and control means connected to said reset means for selectively rendering said reset means inoperative during certain revolutions of said drum, whereby said developing means applies ink to said coating during said certain revolutions, and said printing means prints a plurality of copies of said image.
 13. Apparatus as defined in claim 12 further characterized by the addition of cleaning means for wiping said clinging particles from said coating of said drum.
 14. Apparatus as defined in claim 12 wherein said writing means includes a modulated laser beam.
 15. Apparatus as defined in claim 12 wherein said writing means includes a modulated electron beam.
 16. Apparatus as defined in claim 12 wherein said writing means includes a plurality of slide wires in contact with said amorphous semiconductor coating.
 17. Apparatus as defined in claim 16 wherein an electron beam selectively applies a charge to said slide wires. 